Revert "x86: Side-step lguest problem by only building cmpxchg8b_emu for pre-Pentium"

This reverts commit ae1b22f6.

As Linus said in 982d007a: "There was something really messy about
cmpxchg8b and clone CPU's, so if you enable it on other CPUs later, do it
carefully."

This breaks lguest for those configs, but we can fix that by emulating
if we have to.

Fixes: http://bugzilla.kernel.org/show_bug.cgi?id=14884


Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>